Abstract

The kinetics of the interaction of partially purified bovine erythrocyte acetylcholinesterase (EC 3.1.1.7) with calcium, tetramethylammonium, tetraethylammonium, decamethonium, gallamine, and d-tubocurarine has been investigated, using acetylcholine as substrate. Antagonism between various combinations of ligands has been studied. Decamethonium binds to the catalytic anionic site (α) and to an allosteric site (β). Calcium (0.2 mM) competes with decamethonium but not with acetylcholine, and is considered to act at the β-anionic site. This is an accelerator site, which may bind tetraethylammonium and possibly other polar cations. Tetraethylammonium may also bind to the catalytic site, α. Tetramethylammonium, which is not an accelerator, is considered to bind to the catalytic site exclusively. Neither tetramethylammonium, tetraethylammonium, nor calcium antagonizes the binding of gallamine. This observation, together with that of the partially competitive nature of the inhibition by gallamine, indicates that gallamine cannot bind to the α- or β-anionic sites and hence must bind to a second allosteric site, γ. The ability of gallamine to antagonize inhibition by decamethonium is attributed to allosteric perturbations of the α- and β-sites induced by the action of gallamine at the γ-site.